limiting. As it becomes cheaper to lift material into LEO, lunar sources of low-volume elements that require significant capital investment for their extraction become much less attractive. For the Moon, these constraints imply: (1) Initial space-based resources will be high-volume commodities. (2) Concentration of the desired element(s) in prospective orebodies is extremely important. Even on the Moon, therefore, it will be highly cost-effective to seek out the most concentrated sources of desired materials before committing to an extraction facility. Thus, I believe it is highly unlikely that ordinary regolith will be scooped up at random and separated into its components, as implied by some lunar development scenarios [e.g., Waldron & Criswell, 1982; Binder, 1990]. It has been pointed out that average lunar regolith is about 75% nonfuel "demandite", where "demandite" is defined as a substance containing all elements in the ratios currently demanded by industry. Such an analysis in my opinion is not useful for evaluating lunar resources. It contains two fallacies: (1) Separation is expensive, especially with tightly bound compounds like silicates. As noted, common terrestrial rocks are also mostly "demandite", but they are not economic sources of raw materials. (2) More subtly, "demand" is a function of availability. As prices of a commodity increase, substitutes become attractive in more and more of its applications [e.g., Gordon et al., 1987, pp. 60-76], Designs of space structures will be optimized accordingly. For example, a solar power satellite (SPS) design can be optimized for fabrication from lunar materials [Space Research Associates, 1985]. Again, the high cost of access to space is a two-edged sword: it makes it imperative to "live off the land", but it also makes capital and maintenance expenditures extremely expensive, such that facilities need to be as small and simple as possible. This has been emphasized by Haskin [1985], for example. Such considerations lead to the following general strategy for lunar resource exploration and exploitation. (1) Identify a resource and possible process(es) for extracting it. Such identification can also include tradeoffs with possible by-products (e.g., if ilmenite is to be reduced for O2, Fe might be an attractive by-product.) This constrains the potential ore minerals and thus the host rock types (or regolith types; the lithology of the regolith reflects the underlying bedrock fairly faithfully [e.g., Papike et al., 1982]). (2) Identify possible lunar geologic settings rich in the potential ore minerals, and target them for further examination. Again, terrestrial experience indicates it will be highly cost-effective to do lots of looking first before committing to a mine. Such a procedure is of course iterative, as new processes are developed for
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